The complement system is known for its ability to opsonize bacteria and lyse target cells. However, recent experiments have shown that it also plays an important role in initiating the response to foreign proteins. For example, antigens binding complement component C3d can increase their immunogenicity by up to 1,000-fold, as compared to the same antigen without complement. In this role, the complement system may provide an important link between innate immunity and antibody-mediated specific immunity. We are using this mechanism to enhance the immunogenicity of gp120 by coupling it to C3d. HIV envelope glycoprotein gp120 is heavily glycosylated, which may help the virus to evade immunity by covering up important antigenic sites. We have produced a gp120 mutant which lacks 6 out of 22 sugar sidechains, to expose these sites for immune responses and antibody binding. By itself, deglycosylation of these sites had little effect on either antibody induction or binding. However, to our surprise, the missing sugars had a profound effect on the ability of gp120 to fix complement. In the presence of antibodies, deglycosylated gp120 bound over 100-fold more C3d than did the fully glycosylated form. We are currently studying whether this effect can explain the observation that deglycosylated variants of SIV can be more immunogenic. Monkeys infectd with these variants were able to control their infections much better than could monkeys infected with wild type virus, and they were even able to reduce the viral burden to undetectable. Experiments are underway to measure binding of C3d-gp120 complexes to complement receptor 2 of human B cells, as well as B cell triggering. In addition, a multimeric form of deglycosylated gp120 is being made, so that multiple copies of C3d can bind per complex. These complexes will be tested for enhanced immunogenicity. Complement is thought to enhance immunogenicity by crosslinking complement receptor 2 with the surface Ig on B cells, generating a strong stimulatory signal. Considering that envelope glycoproteins are a common feature among enveloped viruses, these observations may be extended to a wide range of viruses. If the C3d bearing form is more immunogenic, this observation suggests a way to enhance the response to vaccines against a wide range of biologically important pathogens.